1. Field of the Invention
The present invention relates to a reversible thermal recording material and a method of recording an image on the reversible thermal recording material. More specifically, the present invention relates to a reversible thermal recording material that repetitively permits stable recording and erasing of images having a clear contrast at high sensitivity, and a method of stably recording and erasing images repetitively by application of laser beam with a reversible thermal recording material capable of causing a reversible color tone change under the control of heat energy.
2. Explanation of Related Art
In recent years, a reversible thermal recording material is attracting attention, in which an image can be formed temporarily and when the image is no longer necessary, it can be erased. As the above reversible thermal recording material, for example, there have been proposed systems which are prepared by combining a leuco dye and a reversible developer with various light-heat converting materials and which perform recording with near infrared laser beam (JP-A-5-8537 and JP-A-11-151856).
In these systems, it is true that these systems can give a clear image contrast. However, none of these systems have proved formation and erasing of images at high sensitivity and repetitiveness of recording and erasing images 1,000 times or more, which are vital problems in practical application. When the present inventor has made duplicate tests of the above systems, systems using a deposition film of Cr, or the like as a light-heat converting material show low recording sensitivity since the systems cannot be selectively imparted with high absorbance depending upon the wavelength of a near infrared laser beam. For example, the thermal recording material described in JP-A-11-151856 cannot be said to be satisfactory in respect of recording sensitivity. In Examples of the above JP-A Publication, it is described that when a recorded image surface is observed after printing and erasing are repeated 100 times, the image-recorded surface is free of non-uniformity with regard to a recording state and an erasing state and is in a good condition. However, nothing is demonstrated with regard to repetitiveness of 1,000 times or more that is practically required, nor is it guaranteed.
Further, Example of JP-A-11-151856 describes an erasing method based on changing the intensity of laser beam from a laser recording machine. In printing, an image can be printed by applying energy to a desired portion alone. In erasing, however, it is required to apply energy to a large area as compared with the area of an image to be erased, so that the method of merely decreasing the intensity of beam takes a long erasing time period. Further, when the laser beam intensity is changeable for printing and erasing, the system comes to be complicated, and the time period for data processing required for printing and erasing an image comes to be longer, so that there is caused a problem that the erasing and printing come to take a very long time. Further, the above Publication has demonstrated nothing with regard to the applied energy of near infrared laser beam capable of repeating recording and erasing 1,000 times or more.
On the other hand, there is also proposed a method in which a laser beam is applied in a state where a light-heat-converting tape in the form of a cassette tape is in contact with an opaque type thermal recording medium, and printing and erasing are repeated while feeding the light-heat-converting tape (JP-A-2001-315367). In Examples of this laid-open publication, clear images are obtained even when printing and erasing are carried out 1,000 times. However, when the above light-heat-converting tape is used, it is required to exchange cassettes thereof like a cassette for an ink ribbon printer. Further, there is a problem that since the printing and erasing performances are greatly influenced by the state of a contact between the light-heat-converting tape and the surface of a thermal recording medium, they are greatly impaired by the surface roughness of the thermal recording medium and adherence of dust.
Under the circumstances, it is a first object of the present invention to provide a reversible thermal recording material that permits stably recording and erasing images having a clear contrast at high sensitivity a number of times, for example, at least 1,000 times repetitively.
It is another object of the present invention to provide a method of stably recording and erasing images a number of times, for example, at least 1,000 times repetitively with a reversible thermal recording material that causes a reversible color tone change under the control of heat energy by application of a laser beam.
For achieving the above objects, the present inventor has made diligent studies, and as a result, it has been found that the first object above can be achieved by a reversible thermal recording material which comprises a leuco dye, a reversible developer and a light-heat-converting dye, and which uses, as the light-heat-converting dye, (i) a dye whose deterioration ratio under a specific condition is a predetermined value or less or (ii) a phthalocyanine compound having a vanadyl group.
It has been further found that the second object above can be achieved by a method of recording an image on a reversible thermal recording material containing a leuco dye, a reversible developer and a light-heat-converting dye by heat generated by application of laser beam, wherein the energy of the laser beam is controlled to be in a specific range to achromatize the leuco dye that has formed a color.
The present invention has been completed on the basis of the above findings.
That is, the present invention provides
(1) a reversible thermal recording material comprising a colorless or light-colored leuco dye, a reversible developer which causes said leuco dye to develop a color by heating and which achromatizes said leuco dye by re-heating, and a light-heat-converting dye, wherein said light-heat-converting dye has a deterioration ratio of 20% or less when a laser beam having an oscillation wavelength of 830 nm is applied thereto repeatedly 1000 times at a temperature of 25xc2x0 C. at an energy of 2 J/cm2 (to be referred to as xe2x80x9creversible thermal recording material Ixe2x80x9d hereinafter),
(2) a reversible thermal recording material comprising a colorless or light-colored leuco dye, a reversible developer which causes said leuco dye to develop a color by heating and which achromatizes the leuco dye by re-heating, and a light-heat-converting dye, wherein the light-heat-converting dye is a phthalocyanine compound having a vanadyl group (to be referred to as xe2x80x9creversible thermal recording material IIxe2x80x9d hereinafter), and
(3) a method of recording an image by applying a laser beam to a reversible thermal recording material comprising a colorless or light-colored leuco dye, a reversible developer which causes said leuco dye to develop a color by heating and which achromatizes the leuco dye by re-heating and a light-heat-converting dye, to record an image by heat generated by the application of the laser beam, wherein said leuco dye is achromatized with energy that is at least 25% but not more than 65% of the energy of the laser beam applied for causing said leuco dye to develop a color.